From U. of Utah anthropologist Doug Jones’ Logarithmic History
Asabiya and metaethnic frontiersIn 390 BCE an army of Gauls, 30 thousand strong, marched out of northern Italy into Latium, an area that included Rome. They defeated a Roman army, sacked and burned Rome, and left only after being paid a large tribute. This marked a turning point for Rome, which resolved never again to allow such a disaster. Over the next century, Romans used a mixture of coercion and consent to bind their Italian allies more closely to them. Attempted secession was punished. But those who accepted their position as allies were not simply crushed and plundered (as in many other empires) but granted some or all of the privileges of Roman citizenship in return for military contributions. Membership in the Roman confederation was attractive enough that many Italian states sought it voluntarily.The history of Greece during this period is different. Greek city-states never united. In the aftermath of the bloody Peloponnesian war, different city-states went on fighting for supremacy, until they were finally conquered by an outside power, Macedonia.
Note that Macedonia also sometimes portrayed itself as Greek as well.
Peter Turchin is an ecologist-turned-social scientist who thinks that the contrast between Rome and Greece illustrates some general laws of history. According to Turchin, the rise and fall of empires is partly conditioned on the strength of “asabiya,” or social solidarity. (He borrows the term from the medieval Arab historian ibn Khaldun.) The strength of states depends not just on material factors like population size and wealth, but also on morale – on the willingness of citizens to work together for the common good (which includes punishing free-riders). Asabiya was high in early Rome; in Greece, by contrast, while individual city-states might evoke strong group feeling, there was little willingness to cooperate for the good of Greece as a whole.
Other examples would seem to be that the huge German population of Europe eventually coalesced politically under two Eastern “march lord” powers, Prussia and Austria, that had developed militarily to fight Slavs and Balts.
Is this a theory with predictive powers, or is it simply something that happens sometimes but not other times? To test that, you’d need a list of relevant historical events made up for some other purpose, but I don’t have one.